1. Field of the Invention
The present disclosure relates to an electrophotographic type image forming apparatus.
2. Description of the Related Art
Among electrophotographic type image forming apparatuses, so-called tandem type image forming apparatuses are known in which image forming units of different colors are independently provided. Such a tandem type image forming apparatus has a configuration in which images of different colors are sequentially transferred from the image forming units of different colors onto an intermediate transfer belt, and then collectively transferred from the intermediate transfer belt onto a printing medium.
In this image forming apparatus, due to mechanical factors in the image forming units of different colors, color misalignment (position shift) occurs when the images are overlaid. In particular, in a configuration in which each image forming unit includes a photosensitive member and a scanner unit that exposes the photosensitive member to light, constant color misalignment (hereinafter referred to as “DC color misalignment”) occurs due to individual differences in the image forming units. In order to correct the DC color misalignment, the image forming apparatus performs color misalignment correction control. To be specific, a detection developer image (hereinafter referred to as a “detection pattern”) for detecting the position of each color is formed on an image carrier such as the intermediate transfer belt, and the relative position of the detection pattern of each color is detected by an optical sensor, whereby the amount of color misalignment is detected and corrected.
Meanwhile, periodic speed variation occurs in the photosensitive member due to eccentricity of rollers that drive the photosensitive member and the intermediate transfer belt, or other causes. Non-constant color misalignment (hereinafter referred to as “AC color misalignment”) occurs due to the speed variation. The AC color misalignment generates an error in the amount of color misalignment detected in the detection pattern during color misalignment correction control. Under these circumstances, Japanese Patent Laid-Open No. 2001-356542 proposes that detection patterns be arranged at an interval of an integer fraction of the period of the speed variation that is a cause of the AC color misalignment, the number of detection patterns corresponding to the integer. In Japanese Patent Laid-Open No. 2001-356542, color misalignment correction control is performed by averaging the detection results obtained from the thus-formed detection patterns.
Also, during the color misalignment correction control, electrical noise may occur, causing an error in the results of detection of the detection patterns. Furthermore, if an impact or the like occurs due to the operation of the mechanical mechanism in the image forming apparatus while the detection patterns are formed, the positions of the formed detection patterns are shifted. This also causes an error in the results of detection of the detection patterns. Hereinafter, a detection error caused by electrical noise during color misalignment correction control or by an impact or the like of the mechanical mechanism will be simply referred to as “noise”. In order to increase noise resistance during color misalignment correction control, increasing the number of detection patterns is effective.
However, there are cases where the number of patches cannot be increased freely due to a constraint in the arrangement of patches that are developer images constituting detection patterns. Hereinafter, the constraint will be described with reference to FIG. 16.
If, for example, it is assumed that the entire length of the intermediate transfer belt is 600 (mm) and four colors are used in image formation, a requirement is imposed that the maximum length of a detection pattern of a single color is 150 (mm). The detection patterns of different colors each include a plurality of patches. However, if the interval between patches is too short, the accuracy of detection deteriorates. Accordingly, in this case, a requirement is added that, for example, the minimum interval between patches is 15 (mm). As shown in FIG. 16, it is assumed that the length of one period of speed variation that can cause AC color misalignment is 100 (mm), and the maximum value of detection error caused by the speed variation is ΔA. Furthermore, as shown in FIG. 16, it is assumed that noise that causes a detection error ΔB occurs. A detection pattern #1 shown in FIG. 16 is a pattern in which patches are arranged at an equal interval over one period of speed variation while satisfying the requirement that the minimum interval is 15 (mm) in order to reduce the detection error due to AC color misalignment. Specifically, six patches y1 to y6 are arranged at intervals of 100/6=16.67 (mm).
The amount of color misalignment is determined by averaging the detection position or time of each patch, but in the detection pattern #1, the patches are formed over one period of speed variation, and thus the detection error due to the speed variation is cancelled out and is therefore zero. On the other hand, the detection error caused by noise only affects one patch at most, and thus becomes smaller as the number of patches in the detection pattern increases. To be specific, in the detection pattern #1, the detection error caused by noise is ΔB/6. Here, in order to increase the resistance to noise, a case is considered as in a detection pattern #2 in which an increased number of patches are arranged at the same patch interval as that of the detection pattern #1. In the detection pattern #2, three patches y7 to y9 are added at the same patch interval as that of the detection pattern #1.
In the detection pattern #2, the detection error caused by noise is ΔB/9, which is smaller than that of the detection pattern #1. However, the length of the detection pattern #2 is not an integer multiple of the period of speed variation. Here, if the detection error in the amount of color misalignment due to speed variation when the detection pattern #2 is used is determined as (ΔA×2/9) through numerical calculation, the result is worse than that of the detection pattern #1. As described above, when the number of patches is increased in order to improve noise resistance, the detection error due to an AC component may increase. In order to suppress the detection error caused by noise without increasing the detection error due to an AC component, for example, formation of a detection pattern over two periods of speed variation may be conceived, specifically, in the example shown in FIG. 16, formation of a detection pattern so as to have a length of 200 (mm). However, in this example, the maximum length of a detection pattern of a single color is 150 (mm), and thus such a configuration is not possible.